1. Field of the Invention
The invention relates to the stripping of volatiles from reaction mixtures. More particularly, it relates to the stripping of water or other volatile by-products formed during polycondensation reactions.
2. Background of the Invention
An ester product can be formed by the condensation-type reaction of a carboxylic acid with an alcohol, at elevated temperature, as follows: EQU R--CO--OH+HO--R'=R--CO--O--R'+H.sub.2 O
The rate of reaction depends partly on whether the functional groups are primary, secondary or tertiary in nature, on the concentration and properties of the reactants, their solubility, the presence of catalysts and the reaction temperature.
Similarly, polyesterification-involved reactant molecules contain two or more functional groups. With stoichiometric amounts of polyfunctional acids and hydroxyls, polyesterification can theoretically produce resins of almost infinite molecular weight.
Alkyd resins are also produced through such polycondensation reactions. Traditional alkyds are formulated with a significant drying oil and fatty acid content. Alkyds containing oil are typically described as having a given "oil length", which relates to the percent of oil in the alkyl formulation.
Because the polycondensation reaction is reversible, the removal of water is essential to prevent hydrolysis of the ester product. For certain reactants, the removal of water can become the reaction controlling step. Furthermore, the viscosity can change drastically during the reaction cycle, e.g. from 1 cps up to 6,000 cps. Controlling the viscosity during molecular growth is a major concern for coating resin manufacturers. Increases in viscosity also make water removal difficult even at elevated temperature conditions. Process pressures likewise change as the viscosity changes. For example, the pressure drop through an in-line stripping system and 25 feet of piping has been found to increase from about 25 psi at 0% alkyd resin production conversion to about 140 psi at 100% conversion.
The reactants in polycondensation reaction operations can be passed through an in-line stripping system where an inert gas, such as nitrogen, is added to the reactants through an injection nozzle. Such in-line stripping systems are adapted to increase the velocity of the gas-liquid mixture to supersonic speed, i.e. a velocity to above the speed of sound in the mixture, desirably to less than 100 ft/sec., preferably to less than 50 ft/sec., since it is difficult to pump a gas-liquid mixture above 100 ft/sec. As the gas-liquid mixture passes from the narrowest portion of the in-line mixer, e.g. the annular opening between a venturi device and a conical mixer in which the velocity is increased to supersonic range, the subsequent deacceleration to a subsonic velocity creates a sonic shockwave that serves to shatter the gas stream into very fine bubbles, e.g. less than 1/10 of a millimeter in size. Such very fine gas bubbles have a very high surface area desirable for mass transfer.
Water produced in the polycondensation reactions referred to above are stripped from the reaction mixture into such very fine gas bubbles. Because of the very high surface area to volume ratio of the small gas bubbles, the mass transfer of water to the bubbles is very high, and very short residence times within the recycle pipeline of the in-line stripping system are required for the desired water stripping action to occur and for thermodynamic equilibrium to be achieved. Upon entering the polycondensation reaction vessel, the gas bubbles carrying the stripped water are flashed-off from the liquid. Solvent used as reflux is conveniently recovered through a reflux condenser. With the water and any by-product material removed from the reaction mixture, the polycondensation reaction can proceed at a faster speed and with minimum reversible hydrolysis reactions. The desired reaction will, therefore, proceed to completion when essentially all of the water is removed from the reaction mixture by means of such in-line stripping action.
The Cheng patent, U.S. Pat. No. 4,931,225, discloses known in-line stripping systems. While such systems provide for a gas to be injected into a liquid and for the gas-liquid mixture to be accelerated to at least sonic flow velocity by flow area restriction, and subsequent deacceleration to subsonic velocity, such systems are not adapted to handle the variation in process conditions described above. Thus, such systems are adapted for specific flow rate and pressure drop conditions. It is not possible, for example, to change the throat size or the restricted flow area during the course of the polycondensation reaction. Furthermore, each recirculation pump will have a practical operating limit in discharge pressure. Failure to correctly predict the maximum operating pressure can have severe operating consequences. Excess pressure can crack or overload the pump, resulting in process stoppage or undesired reduction in the liquid flow rate.
Even when a stripping system is designed to handle the highest possible pressure, the efficiency of the stripping operation suffers. A stripper that is designed to operate with a highly viscous liquid must have a very large throat size. This is to ensure that the sum of all of the pressure drops in the system will not exceed the capacity of the pump. Such pressure drops can be due to the stripper itself, piping, vertical height, valves, elbows, tees and the like.
For a 4 inch stripper having a throat size of 2.29 inches, using a pump having a maximum delivery pressure of 145 psig, an in-line stripping system can operate, e.g. to produce alkyd resins, at full capacity when the pressure increases to 140 psig at the end of the reaction, e.g. at 100% conversion, due to the resulting increase in viscosity. At the beginning of the reaction run however, the pressure drop through this same stripping system is only 24 psig. An optimum stripper design would have provided for a throat size of only 1.17 inches under such conditions. It can thus be seen that the known in-line stripping systems are not suitable for use with respect to such polycondensation-type reactions, which are subject to very large increases in viscosity during the course of reaction.
It is an object of the invention, therefore, to provide an improved stripping system for the removal of the water of reaction or other volatile by-products from polycondensation-type polymerization reactions, as in the production of polyesters and alkyd resins.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.